1. Field of the Invention
The present invention relates to an electrical resistance measurement method and a component inspection process. The electrical resistance measurement method is for measuring an electrical resistance of a measuring object on which surface a film is formed. The component inspection process employs the electrical resistance measurement method.
2. Description of the Related Art
Conventionally, as a raw material for a casing of a personal computer and a rack for housing the personal computer, metallic plate materials which provide predetermined strength and simultaneously are easy to supply are widely used.
FIG. 1 is a diagram showing an external appearance of a rack for housing an information processing device such as a personal computer.
A metallic plate material is used in a box 30 and doors 31 of a rack 3 shown in FIG. 1. Thus, when the metallic plate material is used in the box 30 and the doors 31 of the rack 3, the metallic plate material also works as a shielding member to suppress unnecessary radiation from the personal computer to the outside. In the rack 3 shown in FIG. 1, if there is a gap between a door supporting section and the doors, the unnecessary radiation leaks out through the gap to the outside when the personal computer generates the unnecessary radiation. Therefore, an endeavor has been made, for example, an unnecessary-radiation preventing member such as an electroconductive rubber 32 is inserted into the gap in order to obtain a better shielding effect than ever.
In recent personal computers, as their clock rates, which determine their operation speeds, have become high and thereby unnecessary radiations at high frequency tend to increase. Accordingly, metallic plate material (or organic plate materials having electroconductivity may be used when their prices become low enough) are used for the casing, the box 30 and the doors 31 of the rack shown in FIG. 1 in order to enhance the shielding performance.
In order to provide a shielding as described above, it is required to evaluate in advance shielding performance of the metallic plate material itself to be a raw material necessary for manufacturing the casing and the rack.
In evaluating shielding performance of the metallic plate material, an electrical resistance of the metallic plate material is measured using a method compliant to the requirement of JIS-C-2550. Then, the material is evaluated to determine that smaller the measured electrical resistance is, higher the shielding performance the metallic plate material has. However, as the metallic plate material used for providing the casing of the personal computer or the rack, a general-purpose zinc-plated steel plate material and the like which are easy to supply are used. Surface treatment such as an anti-rusting treatment different for each manufacturer may be applied to each of those zinc-plated steel plate materials and the like. Further, even though some are supplied from a same manufacturer, different surface treatment may be applied to them. Therefore, when the above-mentioned evaluation is performed, it is required to measure an electrical resistance including the surface treatment to evaluate characteristics including the surface treatment.
However, when electrical resistance of a metallic plate material is measured with a resistance measurement method compliant to JIS-C-2550, a film formed on a surface of the metallic plate material may be pierced through by a sharp-edged probe used in the method, and thus electrical resistances of the steel plate material excluding the film on the surfaces are measured. As a result, almost same resistances are obtained for different metallic plate materials.
Accordingly, a technique is proposed to solve the above-mentioned problem. In Japanese Patent Application Publication No. S57-154069, a technique to measure an electrical resistance without breaking a film on the surface using an electroconductive rubber is proposed. The electroconductive rubber is used in Japanese Patent Application Publication No. S57-154069 as an electroconductive material whose characteristics are almost the same as a metal, and is used to fill a gap between the box unit and the door as shown in FIG. 1. Or, as described in Japanese Patent Application Publication No. S56-79262, this electroconductive rubber is also used as a member which makes it easy to connect a land pattern on a circuit board with a terminal of a leadless component. When the technique described in Japanese Paten Application Publication No. S57-154069 is used, it is possible to evaluate plate materials each manufactured by different manufacturers as well as those manufactured by a single manufacturer. Accordingly, it is possible to readily select a plate material suitable for manufacturing a casing and a rack.
However, when in the technique of Japanese Patent Application Publication No. S57-154069, each of multiple pieces of the electroconductive rubbers is placed for each measurement point to measure each electrical resistance of the plate material with contacting a probe for each of the multiple pieces of electroconductive rubbers, each condition as to how each electroconductive rubber is compressed may vary the electrical resistance between the electrical rubber and the measuring object because each electroconductive rubber is comparatively large and elastic. Accordingly, with the technique described in Japanese Patent Application Publication 57-154069, it is impossible to accurately measure the electrical resistance of the metallic plate material including the film because the state of the electroconductive rubber is not stable.